Small Space Seed Starting Part Two: Potting Up Your Plants

*This post is Part 2 of a series that will take you from the very beginnings of starting seeds in a small space, such as a home, apartment, or backyard greenhouse, all the way through to getting your young plants ready to plant outside. See the other posts in this series for a complete guide on how to start and grow seeds in the small space of a home or apartment (useful for small-scale greenhouse growing, too!)

Now that your seedlings are up, it’s time to start potting up.

What Does It Mean to ‘Pot Up’ Your Plant?

Potting up means pretty much what it sounds like. When you “pot up” a plant, you transplant it into a larger pot. In the case of starting seeds and small-space seed starting, you are going to gently separate those small seedlings and plant the into individual pots so that they can grow to transplant size, ready to be planted outside into your garden, raised bed, or patio containers—whatever your case may be.

Why Pot Up Seedlings?

Seedlings that have been potted up.

Starting seeds in bulk containers helps us get the best, most viable seedlings started to transplant into individual pots and thus, eventually, plants. This method is all at once a quick-start and small-space saver, but more importantly, it is a way to get only viable seeds started, thereby selecting out poor seeds and their future inferior plants in the process. The seedlings were never intended to spend their entire potted lives in these bulk pots. The plan all along was to pot them up for vigorous, individual growth.

The next step, therefore, must be to pot up the plants to give the best of them, our best possible crop, space, soil, water, air, and nutrients to grow.

What Type and Size of Pots to Use to Pot up Plants?

When it comes to pots and options to plant garden transplants in, there are a number of options, many of them good. There’s a good chance you’ve seen lots of ideas, even up-cycled vessels, online that can be utilized. The short version is that most any individual pot or cell that is two to three inches deep by one to two inches wide that has good drainage is adequate.

Whether you choose peat pots that go into the ground, paper cups, “pill” type expanding plant pellets, small single pots, or multi-plant plastic cell packs is up to you, as long as the pot fits the bill for potting up.That said, my recommendation is to use those plastic, multi-cell plant cell packs. If you truly want to avoid plastic, there are biodegradable peat-pot versions of them as well, but I do tend to find more ripping and tearing and degrading before transplant time with these (still not a horrible option).

My preference for plastic cell packs is based on:

  • Ease of use.
  • Reasonable pricing.
  • Compact yet adequate spacing for small-space plant growing.
  • Recyclability—both in that the material is recyclable when it is no longer in good enough condition for planting, but also in that with proper care, cleaning, and storage you can generally use the same set of cell packs for many seasons.
  • Compatibility with standard plant trays.
  • Ease of watering and maintenance with other standard components.

What size plant cell packs are recommended?

804 tray inserts 4-cell pack, daisy tray

One 10×20 plant tray can hold enough cell packs to plant up to 48 individual plants if using a standard 6-cell pack. The same tray will hold 32 plants if using a 4-cell pack.

Either is perfectly adequate and the cost difference is negligible, if at all, so at this point it really comes down to preference and plant quantity.

804 tray inserts for planting

For small-space seed starting, 6-cell packs are probably the most sensible. If you have the extra space, four-cell packs give a little more room for plants to grow without tangling later on.

In the gardening world, 6-cell packs are called 806 tray inserts/cell packs and 4-cell packs are called 804 tray inserts/cell packs.

If you search accordingly, you will come up with them quickly. A full set of inserts usually costs around $5 per tray (32 to 48 cells).

Potting-Up Supply List:

You may have many of the supplies you need already, because many of them will be reused from the seed germinating stage. Now is the time to take a count of how many of each type of plant you are planning for your garden and buy your pots or packs accordingly. You will need:

What type of potting soil to use?

Whereas for germinating seeds we used a specific seed-starting soil, now is the time to move to a labelled potting soil. Potting soil does contain more organics and some fertilizer that the growing plants will need, which seedling soil does not. All you are looking for is a good-quality potting soil that is not too sandy or too filled with large mulch and pieces.

Most good-quality commercial potting soils will fit this bill as long as you don’t go for total bargain-basement substandard soils. It should be a soil that does not compact too much and that contains enough peat or good organics to uptake water and hold moisture. Potting soil is not overly expensive—under $15 for an eight-quart bag, which is enough for most home gardeners, depending on plant quantity.

When to Pot Up Germinated Seedlings

Once your seeds have germinated you should move them to a warm and light place (under your grow lights or near or on a bright windowsill is find, just be sure they are not close so as to be burned by heat-generating grow lights). Generally speaking, you should let these young seedlings grow for one to two weeks before potting them up. This gives them time to grow some reliable roots for the transplant.

Two signs will tell you that your seedlings are developed enough for transplanting.

cotyledon leaves and true leaves

One very reliable sign is when you see the first “true” leaves starting to develop. These are NOT the FIRST leaves you’ll see.

The very first leaves you will see are the cotyledon leaves, which are actually part of the plant embryo and are an early food source for the new seedling. In time you will start to see very small leaflets develop in the center or above these leaves. Seeing these tiny leaves is a sure sign that your seedlings are ready for potting up.

In the above image, note that the round bottom-most leaves are the cotyledon leaves, and the very small, pointed leaves emerging in the middle are the first true leaves.

Some seedlings take quite a while before you see these “true” leaves, and/or the seedlings and leaves are very small, making it hard to discover them. And so, I do not always wait for these leaves before moving ahead and potting up the seedlings of all varieties. As long as the seedlings have more than one single root, where you can begin to see branching in the root, your seedlings should be developed enough to withstand the potting up process.

developed roots in seedlings

To determine root development, you will have to pry up gently on an edge seedling ad take a look at the root structure, then use our own judgement.

(I should say here my personal experience has been that even very young and under-developed seedlings have, for me, successfully transplanted as long as they have lived about two weeks since germination and have some notable roots).

You will have flexibility, even of a couple of weeks, giving you enough time to pot up your plants even if some life something interferes on the ideal week or weekend. Just note that at some point the crowding of the germination pot will stunt seedling growth and waiting too many extra weeks will mean that you are inhibiting the plant’s growth by not separating it and giving it room to individually grow and thrive. So while a little delay is okay, a long delay will delay the development and growth to outside transplant size that you need for future planting and result in delayed harvesting.

How to Handle Seedlings When Potting Up

Always handle seedlings and transplant by the leaves and not the stems when working them. The stems are the weakest part of the plant. You might not expect it, but the leaves and leaf attachment are actually very strong on a plant.

By holding and handling your seedlings by the leaves and never by the stems, you will minimize breakage of seedlings when potting them. You’ll soon come to find that these little powerhouses are much stronger than we give them credit for when properly handled by the leaves and not the stems!

How to Pot Up Garden Seedlings

Step 1. Prep your potting soil.

Prep your potting soil for potting just like you did for germinating seeds. Moisten the soil and loosen it (if compacted) or mix it through. You want the oil to be slightly moist, but not soaked and not clumping.

cell packs in daisy tray

Step 2. Set up inserts and trays.

Assuming you are using cell packs and a daisy tray, insert one set/tray of cell-packs into one daisy tray.

fill with potting soil

Step 3. Fill with soil.

Fill the entire tray of cell packs (or containers if using something else) with soil. Fill even with the top of the pack (soil will compact when watered).

use a popsicle stick or craft stick to pry up seedling bunch

Step 4. Separate seedlings.

Use a pencil, popsicle stick, skewer, or similar tool to gently pry under an entire row or clump of seedlings, then pull them apart as follows:

pry seedlings out of pot gently and then separate

Remove an entire clump or cell of seedlings at one time and then work them apart. You’ll be surprised how easily they come apart without harm if you follow these instructions.

handle seedling by leaves

Handling seedlings individually by their leaves, gently but firmly pull the clumped seedlings apart to separate the roots.

While this sounds harsh and unrealistic, be assured that the seedlings will easily handle this and the roots will separate very easily. Yes, it sounds like they shouldn’t, but they certainly do!

Step 5. Plant into individual pots or cells.

If using cell packs, plant one type of plant per 4- or 6-cell pack, or one plant per single pot. Plant as many different packs and as many different types of plant per tray.
Make a hole in each soil-filled pot or cell (pencil, marker, or just your finger will work just fine for this!). Make the hole fairly deep. The length of the stem and roots will dictate this, but you want to plant the seedlings almost up to the lowest leaves, with just a short section of stem exposed below the leaves, above the soil line.

Seedlings planted inside tend to get long and leggy in the stem, longer than you want them to be for strong stem and plant growth. You will do no harm to the seedlings by planting these young stems deeply. In fact, you’ll do them a favor! This is your chance to correct that legginess!!

plant almost up to leaves

Plant the seedling to a little lower than leaf-level. It helps to think of it this way—the growth of the plant will basically occur above the existing leaves. So, you want that base to be low and sturdy relative to your pot.

This is how I have potted up my seedlings for many years, and it is a method that has always treated me well!

peaking squash seed, newly germinated

Step 6. Label the pack or pot.

Be sure to immediately label each cell pack or pot. Don’t rely on your memory or seedling recognition—things change quickly and many plants and seedlings look similar as they develop!

Step 7. Bottom water with optional diluted fertilizer.

Once you have an entire cell or tray ready, place the pack in a tray of water and bottom water as you did when you started the seeds to germinate. Leave the pack in the water and let it absorb the water from the bottom up. Leave it in the water only until the top of the soil turns evenly dark with moisture, and then remove.

Soak seed pots for better seed starting.

The potting soil you use should have an adequate amount of fertilizer in it to maintain the seedlings for the first couple of weeks, however, it is often recommended, and results in stronger plants, to add a diluted strength of liquid fertilizer to the water when you transplant your seedlings.
This will give them a stronger start, faster growth, and more quickly developed root system.

A good, all-purpose liquid fertilizer will work nicely. Read the instructions to determine the normal dilution strength, and then cut that by 50%. Your plants will need regularly-scheduled fertilization in the weeks ahead.

Step 8. Provide good light.

Plants that have been potted up immediately require good, full-spectrum light that is not too far away, provided for most of the day. If you have a bright, sunny, natural light source you may be able to forego providing artificial light. If you start to see your seedlings becoming very “leggy” and long, or if your plants look yellow or yellow-green, you may need to consider providing an artificial light source, as your natural light source may not be bright enough or strong enough to develop the strong, stocky, free-standing plants that you need for transplanting outside.

Pot Up Until the Potting Is Done!

Different types of seeds germinate at different speeds, and so it may be that you will need to have a few “potting up” sessions until all the seeds are individually planted for your garden.

Once this is done, your young plants will need observation, light, fertilization, and maintenance until the time comes for hardening off and transplanting your young starts out into your garden space.

Enjoy this time as you watch your seedlings grow, and continue to follow along here for more information on maintaining, fertilizing, lighting, and then getting your plants ready for the rigors of outside living!

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